Aircraft icing is one of many hazards pilots can expect to encounter in the course of their aviation careers. Although water on a road or in a puddle will freeze when the temperature drops below zero degrees C., water droplets in free air do not necessarily freeze at and/or somewhat below zero degrees C. When the internal stability of a supercooled water droplet is destroyed due to impacting the surface of an aircraft, its freezing temperature rises. As a consequence, for any flight through supercooled clouds or liquid precipitation at temperatures below freezing, there is a possibility of icing. In order for significant amounts of ice to form on an aircraft, the following two conditions must be satisfied: 1) the temperature of the surface of the aircraft must be colder than zero degrees C., and 2) water droplets present in the air contacting the surface of the aircraft must be supercooled--well below freezing (in a temperature range of between -10 and -40 degrees C.).
Current used methods of icing detection and forecasting in the aviation community, which are slowly becoming, if not already, obsolete, include large area icing forecasting techniques, such as AIRMET, SIGMET, freezing level analysis, the use of satellite images, low-level significant weather prognosis, etc. Icing forecasting techniques typically rely on the assimilation of instrumentation data, which is usually on the order of several to twelve hours old, supplied from a plurality of geographically dispersed ground stations, deciphering the data and then delivering a text report giving a relatively coarse estimate of where icing conditions might be present. The AIRMET and SIGMET forecasting schemes, for example, cover a very large, and therefore coarse, area of at least 3,000 square miles at any one time and are updated only every four to six hours.
The following is a non-limiting example of an AIRMET advisory weather report for a location in Alaska:
AK PEN . . . VALID UNTIL 220800 . . . CLDS/WX . . . PA1 OCNLY MDT ICGICIP 070-140. ENDG BY OOZ. PA1 FRZL VL 070 LWRG TO 040 BY 06Z. PA1 DFWP UWS 051710 SIGMET PAPA 1 VALID UNTIL 052110 PA1 AR LA MS FROM STL TO 30N MEI TO BTR TO MLU TO STL PA1 . . OCNL SVR ICING 90 TO 130 EXPCD. FRZLVL 80 E TO 120 W. CONDS CONTG BYD 2100Z.
This report describes occasionally moderate icing in clouds and in precipitation from 7,000 to 14,000 feet, expected to end by 000Z, and freezing levels at 7,000 feet lowering to 4,000 feet, which are expected to end by 06Z.
Similarly a non-limiting example of a SIGMET advisory weather report for the Arkansas, Louisiana and Mississippi region reads as follows:
According to this advisory, in an area from St. Louis, Mo. to 30 miles North of Meridian, Miss. to Baton Rouge, La. to Monroe, La. to St. Louis, Mo., occasional severe icing can be expected from 9,000 to 13,0000 feet, with freezing levels from 8,000 feet in the East to 12,000 feet in the West. These conditions are expected to continue beyond 2100Z.
Although such forecasting products provide some assistance in determining where icing areas may exist, a pilot must also consider a multitude of other products to identify icing areas. Moreover, since atmospheric conditions are constantly changing, employing instrumentation data that is hours old is not a practical way to provide a necessarily accurate or timely indication of icing. Further, even through past and present usage of satellite, radar and other graphical and textual products provide an approximation of potential icing areas, they fall short in regards to simplicity, accuracy, detail and timeliness.